Dendritic cells are now understood to provide two essential functions in the immune response. In their immature state they capture antigens and can promote tolerance, while in their mature state they stimulate T cells to mount effective immune responses. Maturation is mediated by a variety of stimuli, primarily provided by the pro-inflammatory products produced by microbial pathogens. The current program project application will exploit the novel approaches developed by this research team during the last funding period to test the hypothesis that distinct DC subsets mediate tolerance through distinct mechanisms. These subsets, defined by the CDS, DEC205 and 33D1 surface markers, contribute to tolerance through their ability to lead to the deletion or anergy of autoreactive T cells (the intrinsic pathway) or through their ability to promote the differentiation and expansion of CD25+CD4+foxp3+ regulatory T cells. The collective goals of the program project will be the 1) Manipulation of DC subsets to promote differentiation and expansion of antigen specific CD25+CD4+foxp3+ Treg cells in vivo;2) Regulation FcR signaling on DC subsets as a means of controlling DC maturation and the generation of antigen-specific tolerance and 3) Identification of the role of the DEC205 and 33D1 DC subsets in antigen presentation and tolerance induction. These studies will be pursued by an experienced team of senior investigators with a long history of successful collaborations and productivity, using novel approaches to target DC subsets in vivo and manipulate their maturation. These in vivo approaches with model antigens will then be applied to a series of autoimmune models through the targeting of relevant autoantigens to distinct DC subsets in an effort to restore antigen specific tolerance. The program will be augmented by three core facilities for FACS analysis and sorting, protein expression and autoimmune animal models to promote the research activities of the group. The knowledge gained from this integrated, systematic approach to the function of DC subsets in vivo will provide the basis for the design of novel therapeutic approaches to the treatment of autoimmune disorders.